1. Field of the Invention
The present invention relates to a method of assigning radio communication channels to each of a plurality of mobile stations, in which method a dispersed control system is employed in a dynamic channel assignment system.
2. Description of the Related Art
In a mobile radio system, conventional channel assignment methods include a fixed channel assignment method, in which the assignment of all channels to radio zones is unchanged on a time basis, and a dynamic channel assignment method, in which channels to be assigned are changed on a time basis while the buffer zone is monitored by a central processing unit.
According to the fixed channel assignment method, the following problem occurs: when a new call is generated in a radio zone where a mobile station is busy, only one channel is available; thus the new call is rendered ineffective.
On the other hand, according to the dynamic channel assignment method, for example, when a new call is generated in a radio zone where a mobile station is busy, a channel, which is not used in a buffer zone for the radio zone, can be assigned to the mobile station, thereby preventing the occurrence of an ineffective call and realizing flexible re-use of frequencies.
The dynamic channel assignment method is disclosed, for example, in Published Unexamined Japanese Patent Application No. 2-141036. According to the method disclosed in this document, a control station for controlling base stations collects detailed information relating to the channels used by peripheral base stations. On the basis of the collected information, the channels are checked to select an optimal channel, thus realizing effective use of frequencies. This system is called a dynamic central control system. In this prior art, however, additional control operations are required for assigning channels and to be transmitted among stations. This results in an increase in the system cost and in a delay of connection operation between the mobile station and the base station. These are significant drawbacks in a mobile radio system wherein effective use of frequencies is important.